ToLookup is more forgiving. ToLookup allows null as key. But you may also query a lookup on a key that doesn’t exist, and you’ll get an empty sequence. Do the same with a dictionary and you’ll get an exception.

A dictionary is a 1:1 map (each key is mapped to a single value), and a dictionary is mutable (editable) after the fact.

A lookup is a 1:many map (multi-map; each key is mapped to an IEnumerable<> of the values with that key), and there is no mutate on the ILookup<,> interface.

An overly simplified way of looking at it is that a Lookup<TKey,TValue> is roughly comparable to a Dictionary<TKey,IEnumerable<TValue>>

See example of ToLookup here